In a narrow directional microphone using an acoustic tube, a narrow cylindrical acoustic tube having a prescribed axial length is attached to a front acoustic terminal side, which is a front side of a diaphragm of a unidirectional microphone unit.
Air existing in an acoustic tube serves as an acoustic mass in a low frequency band. The acoustic mass in the low frequency band operates in a manner equivalent to the mass being added to a diaphragm (equivalent to an additional weight to the diaphragm). This facilitates capturing vibration noise.
The acoustic tube is provided with an opening which many sound waves enter (the front end opening or a slit-like opening formed on the wall of the tube). Accordingly, the tube is susceptible to wind noise. Vibration noise and wind noise mainly include low frequency components.
For the purpose of reference, FIG. 4 shows a graph of directional frequency response of a narrow directional microphone using a conventional acoustic tube. This graph indicates that an output level at a low frequency band increases and vibration noise and wind noise largely appear.
Thus, the present assignee has proposed a technique that covers the front end opening of an acoustic tube with a film capable of being displaced by sound waves, mainly for reducing wind noise, in Japanese Patent No. 4684012.
This technique allows the film to prevent low frequency sound waves from passing. Accordingly, wind noise can be reduced. However, in the case where the film is planar, bending of the film by a wind or the like sometimes makes noise that is specific to the film. The invention described in Japanese Patent No. 4684012 uses a film preferably formed into a corrugated shape.
However, even in the case where the film is formed into a corrugated shape, the film has a mass and a stiffness to restore the film to the original position. Accordingly, in a narrow directional microphone having the configuration described in Japanese Patent No. 4684012, resonance occurs owing to the stiffness of the film and the acoustic mass of an air column in the acoustic tube. In an equivalent circuit, the stiffness is represented by a capacitance C, and the acoustic mass is represented by an inductance L.
FIG. 5 shows a graph of directional frequency response measured by the narrow directional microphone having the configuration described in Japanese Patent No. 4684012. In this measurement example, resonance due to the C and L occurs around 200 Hz, showing degradation in directional frequency response. This also means degradation in sound quality.
It is thus an object of the present invention to provide a narrow directional microphone including an acoustic tube having a front end opening covered with a film mainly for reducing wind noise wherein degradation is reduced in directional frequency response due to resonance between the stiffness of the film and the acoustic mass of the air column in the acoustic tube.